i o TAB ULA TE CORALS. 



cannot .be precisely and in all cases homologous structures. 

 With regard to the " tabulse " in the true Actinozoa, Professor 

 Verrill concludes that " they are formed after each discharge of 

 ova : the vacuity thus produced, being useless, is cut off from 

 the visceral cavity above it by the formation of a septum. 

 Therefore, if the eggs be discharged from all the radiating 

 chambers simultaneously, or if from any other cause the polyp 

 abandons all the chambers simultaneously, it is obvious that a 

 complete septum or transverse plate will be formed across the 

 entire tube ; but if the' eggs be discharged at different times 

 from the ovaries occupying the various radiating chambers, the 

 septa formed below them in the different chambers will not be 

 coincident, or exactly at the same level in all. It would seem, 

 therefore, that the existence or non-existence of complete trans- 

 verse plates is simply a matter of periodicity in the discharge 

 of ova" (Amer. Journ. Sci. and Arts, sen 3, vol. iii. p. 187). 

 Dr Lindstrom has further adduced a considerable body of evi- 

 dence in support of the view that the " tabulse " are only a 

 modification of the endothecal " dissepiments " (Ann. and Mag. 

 Nat. Hist., sen 4, vol. xviii. pp. 2, 3). That this is the case in 

 certain instances (such as Ccelastr&a, Verrill) seems quite un- 

 questionable; and it is indeed easy to see how the simultaneous 

 production of dissepiments in all the intermesenteric chambers, 

 at one level, would give rise to a structure entirely undistin- 

 guishable from a " tabula." There are, however, some consid- 

 erations which should not be lost sight of, before it is concluded 

 that the " tabulse " of the tabulate Actinozoa are merely modified 

 interseptal dissepiments. Thus it is to be remembered that in 

 the Rugose Corals there is generally a simultaneous develop- 

 ment of both these structures, and that the " tabulse " are best 

 developed in the axial region of the visceral chamber, into 

 which the septa either only penetrate slightly (Diphyphyllum) 

 or do not enter at all (Amplexus), and in which, therefore, 

 " dissepiments " are either scanty or totally absent. Again, in 

 various members of the Favositidcz, the septa are obsolete ; and 

 there are therefore necessarily no " interseptal dissepiments," 



